The objective of this invention is to provide improved vibration damping performance to metallic substrates. Examples of such substrates include, but are not limited to, those used for the construction of vehicles. More specifically, the objective of this invention is to provide improved vibration damping within the range of temperatures frequently encountered during driving, namely from −30° to 50° C. and most frequently from −10° C. to 40° C. Another objective of this invention is to provide improved vibration damping within this temperature range across the frequencies audible to humans, particularly in the low frequency range of 10 to 200 Hz as described in “Low Frequency Noise. What we know, what we do not know, and what we would like to know”, Leventhall, Geoff, Journal of Low Frequency Noise, Vibration and Active Control 28, 2, pp. 79-104 (2009).
The reduction of noise, vibration, and harshness (often abbreviated as NVH) to humans is a goal of many industrial processes. Exposure to NVH comes from numerous sources, and can be mitigated by various means. For example, laminated safety glass can be comprised of acoustic interlayers which suppress sound transmission. Applications of such acoustic interlayers can include glass panes in commercial and residential buildings and automotive glazing. Other sources of NVH in vehicles include engine noise, road noise, springs and suspensions, braking, and chassis vibration. Noise suppression techniques include component design to reduce vibration and sound transmission; use of composite materials instead of metals; elastomeric sleeves or guards; nonwoven fabrics; carpet or other materials applied to the vehicle interior; foam; liquid-applied damping formulations; and objects produced from viscoelastic materials, such as bitumen or asphaltic pads. Although effective to varying extents depending on the source of the noise, these techniques suffer from limitations. For example, asphaltic pads cannot easily be placed and conformed to some locations on a vehicle body, require manual application, are subject to embrittlement, and must continue to adhere to the metal substrate in order to be effective. Some materials contribute undesired weight to the vehicle, contrary to weight reduction goals designed to improve fuel mileage. Materials which require high temperature and/or long times to cure can slow production, add cost, and result in higher energy usage.
One mode of NVH is through vibration. Polymeric materials can damp, or reduce oscillations of, a substrate by dissipating the oscillation energy with their viscoelastic behavior. A standard measurement of damping utilizes the Oberst method and apparatus. In this method, a material engineered to confer damping behavior is affixed to a stainless steel bar which has negligible damping itself. The effect of the damping material is deduced from the behavior of the sample bar compared to an untreated reference bar. Damping behavior may also be measured using Dynamic Mechanical Thermal Analysis, or DMTA. In this technique, a sample is exposed to a sinusoidal force, generally over a range of temperatures or frequencies. When heated, the modulus of a viscoelastic polymeric substance varies greatly from the glassy state at low temperatures, through the glass transition to a rubbery state, and finally to a lower viscosity molten state. The ratio of the storage modulus to the loss modulus, a value known as the tan δ, is a measure of the material's ability to damp vibrations. Higher tan δ values signify more effective damping behavior. The DMTA tan δ has been shown to correlate well with the Oberst bar testing.
Plasticized polyvinyl chloride (PVC) is well known in the automotive industry. Plasticized PVC applied as a plastisol in automotive underbody coatings and sealants, after thermal curing, can protect the vehicle from chipping by stones and other materials on the road surface. Such coatings also offer protection against corrosion, for example from salted roads. These coatings are typically formulated with general purpose plasticizers and also provide a low level of reduction of the transmission of vibrations from metallic substrates. However, the performance of these plasticized PVC coatings is inadequate to confer satisfactory vibration damping across the range of temperatures and noise frequencies typically encountered without the incorporation of additional damping techniques. These performance deficiencies are exacerbated when the desire to reduce NVH to vehicle passengers over traditional levels is considered. Despite these deficiencies, the ease of application and economy of PVC plastisols make them an appealing potential solution to the reduction of NVH should performance improvements be realized. There is a need to be able to improve plasticized PVC coating damping performance using formulations comprising general purpose plasticizers.